This could be a problematic thing. I remember reading 2010: Odyssey Two, Arthur C Clark, and though we were told not to go Europa, people went, and someone got eaten, and the thing that ate him died within a few minutes due to not being compatible.

However, I would imagine aliens capable of interstellar travel would know this. They might bring their own food. The same way we might take some food if we go to the moon. I would assume people going interstellar distance would have more than just food supplies, though. They're probably making food.